A wellhead assembly, such as that employed on the seabed for offshore drilling and production operations, may often include a tree spool used to access the well bore. The tree spool may be a dual bore spool providing access to both a production bore and an annulus bore. A tree cap may be used to both seal off the bores as well as allow access to the tree spool. One or more seals are typically disposed between the tree cap and the tree spool.
The installation and retrieval of a tree cap on a subsea tree spool can be difficult due to subsea depths and the potential heavy weight of the tree cap. It would thus be beneficial to provide a light weight tree cap to facilitate the installation and retrieval of the tree cap by a remote operated vehicle (ROV). The tree cap may have integral buoyancy to regulate wet weight. The tree cap may have integral buoyancy to help the underwater transport of the tree cap. For example, a buoyant material such as foam may be attached to reduce the underwater weight of the tree cap.
One problem that may be encountered when installing a tree cap with an ROV is properly aligning the tree cap with the tree spool. It thus may be beneficial for a tree cap and tree spool that are configured such that the tree cap is properly aligned on the tree spool. For example, the tree spool may include a means for properly aligning the tree cap on the tree spool, such as a funnel structure and/or a pin that engages a recess on the tree cap.
Another problem that may be encountered when installing a tree cap is properly securing the tree cap onto the tree spool. It would be beneficial to provide deflectable collet fingers that are adapted to snap into a recess when the tree cap has been landed. The deflectable collet fingers may secure the tree cap to the tree spool while a hydraulic plate, cylinder, or piston moves stabs into the bores. The hydraulic plate, cylinder, or piston may be used to overcome the outward force of the deflectable collet fingers. It may also be desirable to provide a secondary locking mechanism that secures the tree cap to the tree spool.
Fluid may become trapped within the cavity of the tree cap while the tree cap is installed onto a tree spool. This may be problematic as the fluid may cause corrosion, encourage microbial or algae growth, or otherwise prevent the tree cap from properly landing on the tree spool. It would be beneficial to provide means, such as a port, for releasing trapped fluid from within the cavity of the tree cap. It would also be beneficial if this means allowed for injection of fluid, such as corrosion inhibitor, biotoxin, or hydrate formation inhibitor, within the tree cap. Further, it would be desirable if this means also allows the production bore and/or annulus bore to be flushed prior to removal of the tree cap from the tree spool. It would further be desirable to provide another means, such as a port and valve in the tree spool, for flushing the production bore and/or annulus bore prior to removal of the tree cap from the tree spool. It would also be desirable to provide a tree cap that provides for hydraulic isolation between the production bore and the annulus bore of a dual bore subsea tree.
Another problem that may be encountered when installing a tree cap is preventing damage to the seals within the tree cap. It is important that the seals within the tree cap are protected as the tree cap is installed onto the tree spool. Energizing the seals after the tree cap has been locked onto the spool may be one way to ensure the seals are not damaged during the installation of the tree cap onto the tree spool. A hydraulic plate, cylinder, or piston may be used to engage production and annulus stabs into their respective bores to energize seals within the bores. It would also be beneficial to provide means to pressure test the seals after the tree cap has been landed. Providing a tree cap that includes a port, or combination of ports, through which the ROV can pressure test the seals may be one way to test the seals after the tree cap has been landed. It may further be desirable to provide a tree cap with a plastic cap on the ends of a production stab and an annulus stab to protect the tree cap and the tree spool while the tree cap is landed on the tree spool.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the issues set forth above.